SUMMARY/ABSTRACT The long-range objective of the proposed research is to determine the cellular and molecular events that lead to the differentiation of specific cell types in the vertebrate retina. We continue our emphasis on the roles of extracellular factors in regulating photoreceptor diversity and differentiation, as the work of the current funding period has demonstrated the significance of these factors not only for photoreceptor differentiation, but also for photoreceptor fate and for photoreceptor maintenance. Our work has provided in vivo evidence that the extracellular factor retinoic acid (RA) influences rod vs. cone neurogenesis when supplied to late retinal progenitors. In addition, RA selectively regulates the differentiation of specific photoreceptor populations when supplied later in retinal development. These regulatory functions are distinct from those of the extracellular factor Hedgehog (Hh), which is required for differentiation of all photoreceptor types, and which is required throughout the lifespan for cone photoreceptor maintenance. In the proposed award period, we will build on these studies through the evaluation of specific extracellular factors on photoreceptor fate and differentiation, examining cell-selective effects on networks of genes involved in rod and cone determination, differentiation, and in photoreceptor pathology. We apply a combination of genetic, molecular, pharmacological, histological, computational, and bioinformatics tools to the zebrafish model. We will test the following hypotheses:1) that RA and Notch signaling control rod vs. cone fate; 2) that the microenvironmental factors RA and Hh selectively manipulate cell-specific gene networks during photoreceptor differentiation; and 3) that limiting quantities of Hh signaling throughout the lifespan will engage a photoreceptor damage response.